1. Field of the Invention
The present invention relates to a low frequency power converter and specifically to low frequency electronic ballasts for gas discharge devices. More specifically, the present invention relates to a low frequency square wave electronic ballast for high pressure sodium lamps.
2. Prior Art
An important application for high frequency switchmode power converters is supplying power to gas discharge devices, especially high pressure sodium (HPS) lamps. In the case of high frequency powering of gas discharge lamps, the high frequency ballast and the gas discharge lamp have a higher level of interaction than that which exists between a conventional low frequency ballast and gas discharge lamp. High frequency ballasts suffer from acoustic resonance which can cause various problems such as instability, high output fluctuation, or, in the worst case, cracked arc tubes. Therefore, an optimum solution to this problem is the use of a high frequency DC-to-DC switch-mode as a controlled current source connected to a low frequency DC-to-AC square wave inverter supplying the gas discharge lamp. Due to its lessened weight, higher efficiency and the nonexistence of acoustic resonances, this novel high frequency ballast with a low frequency output has significant advantages when compared with either the conventional low frequency ballasts and the usual high frequency electronic ballast. Additionally, a new, high sophisticated electronic ballast generation can be introduced to provide several special features, such as, for example, automatic or controlled dimming.
Therefore, the essential design targets for a highly sophisticated, high efficiency, low frequency electronic ballast supplying gas discharge lamps would be the following:
(a) very high efficiency (.apprxeq.95% ), energy saving and low temperature rise of critical components; PA1 (b) low frequency square wave lamp current (crest factor is unit). Since the instantaneous lamp power is constant (no flickering), no acoustic resonance can occur; PA1 (c) high reliability and long life time (lower power loss, no electrolytic capacitors); PA1 (d) fully controlled lamp power, eliminating the effects of line voltage fluctuation and lamp aging; PA1 (e) programmed and/or controlled dimming providing essential energy saving; PA1 (f) automatic switching off when the lamp has reached the end of its life time (programmed in advance); PA1 (g) automatic light switching and high temperature protection; and PA1 (h) power factor correction (PF.gtoreq.95% ) and EMI filtering.
The prior art is replete with many known circuits providing high frequency ballast for gas discharge lamps. For instance, high efficient electronic ballast which can be used with HPS (HID) lamps are discussed in U.S. Pat. No. 5,097,183 entitled "Master-Slave Half-bridge DC-to-AC Switchmode Power Converter" and U.S. patent application Ser. No. 883,762, filed May 15, 1992, entitled "Self-Symmetrizing and Self-Oscillating Half-Bridge Power Inverter" assigned to the assignee of the present invention.